1. Technical Field
The present disclosure relates to high frequency imagers, for example terahertz imagers, formed from a pixel matrix.
2. Description of the Related Art
Terahertz imagers are devices adapted to capture the image of a scene based on terahertz waves, i.e., waves having a frequency that is for example comprised between 0.3 and 3 THz. A conventional imager such as disclosed in the U.S. Patent Application Publication No. 2014/070103 of the applicant includes a terahertz waves emitter for illuminating a scene to be imaged, and a sensor made of a pixel matrix that receives terahertz waves from the scene. Terahertz imagers are used in a large number of applications in which it is wished to see through some materials of a scene. Indeed, terahertz waves penetrate a large number of dielectric materials and non-polar liquids, are absorbed by water and are almost entirely reflected by metals. Terahertz imagers are in particular used in security scanners in airports to see through the clothes of a person or through luggage so as to detect metallic objects for example.
FIG. 1 is a reproduction of FIG. 1 of U.S. Patent Application Publication No. 2014/070103. The sensor 1 includes a matrix 3 of pixels 5 adapted to capture terahertz waves. A row decoder 7 receives a row selection signal 9 that indicates which row is to be read and provides to the lines of the matrix 3 a corresponding control signal 11. The pixel matrix 3 provides output signals 13 for each column of the matrix. The output signals 13 are coupled to an output block 15 that selects and controls each column. The reading of the columns is controlled by a column decoder 17 coupled to the output block 15 and, in this example, the columns are read the one after the other. The output block 15 provides an output signal 19 representing the value of the pixel 5 of the selected row and column. The output signal 19 is amplified and coupled to an analog to digital converter 21.
To analyze the received signal, this signal is combined with a reference terahertz signal provided by an oscillator 23. The oscillator 23 is disposed outside of the matrix 3 and provides a same terahertz signal to a large number of pixels or to all the pixels of the sensor 1. This oscillator 23 is preferably coupled with a terahertz emitter, not shown, illuminating the scene to be analyzed.
FIG. 2 is a reproduction of FIG. 3 of US application No 2014/070103 and illustrates an example of one pixel 5 of the sensor 1. The pixel 5 comprises a detecting antenna 25 and a detection circuit 27 formed, in this example, of two N-MOS transistors 29, the gates of which are biased at a potential Vgate. The antenna is coupled to the oscillator 23 shown in FIG. 1 and to the detection circuit 27. The output of the detection circuit 27 is coupled to a row and column selection circuit 31. The selection circuit 31 is controlled by a signal RSEL provided by the row decoder 7 of the sensor 1 and by a signal CSEL provided by the column decoder 17 of the sensor 1. The analog output signal 19 representing the value of the pixel 5 is available at a node COLOUT that is coupled to the converter 21 (FIG. 1) of the sensor 1.
FIG. 3 is a reproduction of FIG. 5 of US application No 2014/070103 representing an example of a frequency oscillation circuit 33 of a terahertz imager. The circuit 33 comprises a ring oscillator made of an odd number N of inverters, three in this example. Each inverter includes a NMOS transistor 35 the drain of which is coupled to a node 37 and the source of which is coupled to ground. Each node 37 is coupled through an inductor 39 to the gate of the next transistor 35, the inductors 39 having a same inductance value. Each node 37 is further coupled to a summation node 41 through an inductor 43, the inductors 43 all having the same inductance value. The summation node 41 is coupled to a DC voltage source 45 via an inductor 47 and to an output node 49 of emitter 33 via an inductor 51. As shown, the output node 49 can be grounded, for example through a resistor 53.
In operation, the signal generated by the ring oscillator has a fundamental sinusoidal component of frequency F and harmonic sinusoidal components one of which has a frequency N*F. The value of each inductor 43 is selected to implement a band-pass filter centered on the frequency N*F, and an output signal having a frequency fL0 equal to N*F is available at the output node 49 of the emitter 33 that is coupled to a terahertz emission antenna.
FIG. 4 is a partial reproduction of FIG. 8 of US application No 2014/070103 and schematically illustrates an example implementation of the frequency oscillation circuit 33 as disclosed in connection with FIG. 3, but with five inverters instead of three. In this example, each inductor 39, 43, 51 is implemented as a transmission line.
The terahertz imager disclosed in connection with FIGS. 1-4 is a far-field imager provided for seeing through some materials of voluminous objects, seen at a far distance from the object, having sizes greater than 10 cm, preferably greater than 1 meter. The resolution of an image obtained with a far-field imager is at best of about the operating wavelength of the imager, i.e., 1 mm at a frequency of 300 GHz and 0.1 nm at a frequency of 3 THz. To improve the spatial resolution of a far-field imager it is possible to increase the operating frequency of the imager. However, this raises various problems. Thus, a far-field terahertz imager is not adapted to obtaining an image having a resolution in the order of tenths of a micrometer.
Near-field terahertz imagers provide an image of an object to be analyzed with a resolution in the order of tenths of a micrometer. However, these imagers are complex to implement, in particular due to the fact that they use terahertz emission sources such as coherent synchrotron radiations, and optical systems such as elliptical mirrors. An example of such a near-field imager is disclosed in the article “THz near-field imaging of biological tissues employing synchrotron radiation” of Shade et al., published in 2005 in Ultrafast Phenomena in Semiconductors and Nanostructure Materials IX, 46.
Thus, it would be desirable to provide a near-field terahertz imager that is as simple as possible and that provides an image having a resolution in the order of tenths of a micrometer.